Development of soft-switching power supplies began strongly in the decade of the 1980's and has continued meanwhile. Single-stage consolidation including PFC correction began mainly in the 1990's. Many resulting designs have appeared in patents, and others have been made the subject of publication in technical journals. Yet significant trade-offs are inherent in known designs, such as voltage stress upon the customary energy storage ("bulk") capacitor as power factor approaches the desired value of unity.
Such a trade-off is a common problem in such designs, including my own contributions, as well as those of other contributors to this art. Representative examples appear in consolidated or single-stage AC/DC converters patented by such noted designers as the following:
Fraidlin, Slack, and Wadlington in U.S. Pat. No. 5,115,182 (1992) for Single Conversion Power Factor Correction using SEPIC Converter, providing a design with unity power factor but with low-frequency ripple in the output and with slow transient response; and
Teramoto, Sekine, and Saito in U.S. Pat. No. 5,301,095 (1994), for High Power Factor AC/DC Converter, a PFC corrective design with a sole high-frequency capacitor but wherein the primary diode undergoes hard switching unsuited to high-frequency operation, and with input and load ranges narrow without extensive frequency modulation; and
Tsai, Poon, Ho, and Lee in U.S. Pat. No. 5,652,700 (1997), for Low Cost AC-to-DC converter Having Input Current with Reduced Harmonics, a single-stage PFC AC/DC converter with multiple primary windings on an isolation transformer (a primary in the rectified positive lead) in the converters in the first dozen drawing sheets--but not in the featured final (13th) sheet or in any of the claimed circuitry; their energy storage or "bulk" capacitor is so voltage-stressed that a 450 V AC rating is needed for a universal (90 to 264 VAC) input range to obtain even an 0.8 power factor, and subject to a pulsating input current, with high switching AC ripple requiring optimal EMI filtration, and its primary diode undergoing hard switching; and
Brkovic & Cuk, in U.S. Pat. No. 5,642,267 (1997), for Single-Stage, Unity Power Factor Switching Converter with Voltage Bidirectional Switch and Fast Output Regulation, a single-stage PFC AC/DC Cuk converter with a saturable inductor ("magnetic amplifier") as a bidirectional switch to improve the input current waveform, although capable of providing perfect power factor, having its input current operating in deep discontinuous mode, requiring a large EMI filter, with low efficiency owing to high switch-current stress, plus a costly and complicated controller including multipliers and dividers; the bulk capacitor voltage stress, although tightly regulated over the entire line and load range, has to be significantly greater than the maximum peak rectified line voltage (above 450 V for universal input).
Notable examples of contributions to technical (but not patent) literature in this general field have included the following:
Madigan, Erickson, and Ismail, Integrated High Quality Rectifier-Regulators, PESC '92 Record, about BIBRED (and BIFRED) designs having very good power factor and regulation but relatively poor efficiency and excessive voltage stress on the bulk capacitor; PA1 Redl, Balogh, and Sokal, A New Family of Single-Stage Power-Factor Correctors with Fast Regulation of the Output Voltage, PESC '94 Record, teaching power factor correction with a sole inductor, but imposing high switching stresses because of deep discontinuous conduction operating mode and requiring a large EMI filter; and PA1 Huber and Jovanovic, Single-Stage, Single-Switch Isolated Power Supply Technique with Input-Current Shaping and Fast Output-Voltage Regulation for Universal Input-Voltage-Range Operations, IEEE APEC 1997 Proceedings, describing a converter reasonably capable of 0.9 power factor with a 450 V capacitor but necessitating a complicated power transformer with at least three primary windings, depending heavily upon leakage inductance, very resistant to adequate control. PA1 Jitaru, in U.S. Pat. No. 5,126,931 (1992) for Fixed Frequency Single Ended Converter Switching at Zero Voltage; Farrington, Jovanovich, and Lee in U.S. Pat. No. 5,325,283 (1994) for Novel Zero-Voltage-Switching Family of Isolated Converters; and also Vinciarelli, in U.S. Pat. No. 5,432,431 (1995) for Boost Switching Power Conversion Using Saturable Inductors.
A U.S. patent to issue (in 1998) upon my application Ser. No. 08/657,615 for Consolidated Soft-Switching AC/DC Converters discloses a family of converters able to provide good power factor, good efficiency, and also continuous input current while maintaining reasonably low energy-storage capacitor voltage stress, but also subject to such a trade-off between bulk capacitor voltage stress and power factor, whereby the higher the power factor, the greater the voltage stress.
Accordingly, my present effort is to improve further upon the design of single-stage AC/DC converters, especially in that regard.